Solar cells are used to convert sunlight into electricity using a photovoltaic effect. As shown in FIG. 1a, solar cell modules 100 on the basis of crystalline silicon solar cells may typically include 6×10 solar cells 104 of dimensions 15.6×15.6 cm2 which may be arranged in six parallel interconnected solar cell strings. Each string may include ten or twelve mono- or multi-crystalline solar cells that are connected in series by copper ribbons 106. The strings in turn may be again connected in series by so-called cross-connectors 105 so that all cells in the module are connected in series. Solar cell modules with for example 4×9, 6×8 or 6×12 solar cells in the same type of configuration are also common.
Under normal operation conditions, all solar cells may be illuminated and operate at their maximum power point of about 0.5 V. The total module voltage thereby adds up to about 30 V for a solar cell module of 6×10 solar cells. Under certain circumstances, however, partial shading of the module can occur. When a solar cell is shaded, the generated electrical current decreases proportionally with the illumination level. Due to the series connection, the cell with the lowest current determines the overall current in the module. In a situation with only one cell being shaded, this would lead to a complete loss of power of the whole module.
To avoid such complete power loss, so-called by-pass diodes 101 may be incorporated into the module. The by-pass diodes are connected in parallel with a certain number of solar cells. In case of shading, only the cells that are in parallel with the same by-pass diode as the shaded cell may be affected by the power loss. The number of by-pass diodes per module is a compromise between the number of cells that should be affected by partial shading and the cost for incorporating the by-pass diodes. Typically, two strings including 20 cells maximum are connected to one by-pass diode. The by-pass diodes may be located in a junction box 102 that serves as a fixture for the cables used to connect the module to neighboring modules. FIG. 1b shows the electrical schematics of a typical module 100 with three by-pass diodes 101 that are mounted in the junction box 102. The strings are connected to the junction box by the cross-connectors 103 and are connected in series with each other by the cross-connectors 105 on the opposite side.
In a partial-shading situation, where only one cell 104 is completely shaded, the by-pass diode short-circuits all cells that are connected in parallel to the diode. In that situation, the illuminated cells still operate between their maximum power point and their open-circuit voltage at about 0.5-0.6 V each, whereas the shaded cell does not generate any voltage. In contrast, the combined voltage of the illuminated cells of 19 times about 0.5-0.6 V leads to a voltage of up to about 11.4 V being applied to the shaded cell in reverse bias direction.
Due to the diode-characteristic of the solar cells, there is only a negligible reverse saturation current flowing when a reverse bias voltage is applied. However, the solar cell can only withstand a certain maximum reverse bias before it comes to avalanche breakdown of the diode which may lead to rapid heat generation and ultimately to the destruction of the solar cell. Even before destruction, local shunts or “hot-spots” may lead to increased heat generation that can damage the module encapsulation and even cause fire.
Therefore, the maximum applied reverse bias voltage should not exceed the breakdown voltage of typically about 13 V. The exact breakdown voltage depends on the wafer material and the cell design of the solar cells. At given open-circuit voltages of the solar cells, the breakdown voltage limits the number of cells that can be connected to one by-pass diode.
The numbers above show that in the conventional module layout with cross-connectors and junction box at the narrow side of the module, the number of cells per by-pass diode is already close to the maximum.
An approach to increase module output power is to reduce the length of the solar cells in the direction of their interconnection with the ribbons 106, typically achieved by cutting the cells in half. By doing so, resistive losses, which show a parabolic dependence on the length of the cells, can effectively be reduced. Power output can be improved by around 2% with such an approach. However, the number of cells in each string doubles and so does the number of cells per by-pass diode.
Another approach may be using half-cut cells and using one by-pass diode for each string, ie, by incorporating a connector ribbon to connect the one end of the string with the junction box on the opposite side. The drawback of this solution is the power loss of approximately 0.5% in the connector ribbon and the substantial additional cost for the ribbon as well as the necessity to provide multiple back sheet layers where the ribbons are located to avoid shunting.
Therefore, there is a desire to have an optimal cell configuration in solar cell modules such that the maximum reverse breakdown voltages are not exceeded and such that the use of connector ribbons of excessive lengths is avoided.